Fusion jointing of plastic pipes

ABSTRACT

A process for fusion jointing or joining of pipe sections includes the steps of covering planed ends of the pipe pieces with an electric socket. The electric socket has a heating device disposed therein, and a heating output of the socket is such that an effective butt-jointing occurs between the pipe pieces. A bellows can be placed within the pipes to prevent collapse thereof, and to improve an inner bead quality of the pipe parts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for the fusion jointingor joining of pipe parts having a small diameter, and being made of amaterial such as polyvinylidene difluoride (PVDF). Pipes made of otherplastics, such as polyvinylchloride (PVC), clean polyvinylchloride(CPVC), polyethylene (PE), polypropylene (PP), ethylenechlorotrifluoroethylene (ECTFE), perfluoroalkoxyalkane (PFA), polyetherester ketone (PEEK), polyphenylene sulphide (PPS) or the like can alsobe jointed to one another according to the invention.

2. Description of the Related Art

The use of so-called electric sockets in the fusion jointing of pipesmade of high-density polyethylene (HDPE) is known in the art. In thisoperation, the axially aligned pipe parts to be fusion-jointed arelinearly aligned, maintaining a certain gap at a butt joint into which aring made of the same material as the pipe parts to be fusion jointedcan also be inserted. The electric socket is arranged around the buttjoint, and heated by means of the electric heating element held therein.This causes a softening of the plastic at the socket/pipe partsinterface, and consequently a fusion jointing of socket and pipe parts.However, a smooth fusion jointing does not take place between the pipeparts along the pipe gap on the inner diameter of the pipe. The gapremains in practice and consequently contributes to a relativelypronounced disturbance of the flow which will occur in the jointed pipeparts during use. In addition, in many cases the electric heatingelement is exposed in the region of the pipe gap and is thereforesubject to corrosion.

Until now, the technique described of fusion jointing in HPDE pipelineconstruction with comparatively large pipe diameters could not be usedin the fusion jointing of PVDF pipeline systems with their relativelysmall diameters. In the case of PVDF pipes, owing to the small diametersmentioned and the associated small wall thicknesses, when the knownelectric-socket fusion jointing is used, the pipe parts tend to collapsewhen heated, which has the effect that a reliable, durable, reproduciblefusion joint quality cannot be obtained.

SUMMARY OF THE INVENTION

An object of the invention is to provide a process of the type mentionedat the beginning by which electric-socket fusion jointing can be usedsuccessfully for the fusion jointing of in particular PVDF pipes andpipe-like attachment pieces made of such plastic. In addition, a socketis disclosed, by which the process according to the invention can beimplemented.

This object is achieved according to the invention in that the plasticspipe parts to be fusion-jointed are butted against each other, and theplastics pipe parts are heated over a relatively wide region of the buttjoint over a length of in each case about 0.2 to 1.0 times the pipediameter, measured from the butt joint, by means of an electricallyheated socket made of plastic and are fusion jointed to the latter, thedirect or relatively narrow region of the butt joint being heated moreintensely than the relatively wide region, so that there occurs not onlythe fusion jointing of the inner surface of the socket and the adjacentsurface of the pipe parts but also a butt fusion jointing of the pipeparts along the butt joint.

The process according to the invention consequently provides for, interalia, an intensified heating of the region of the butt joint of bothpipe parts, it having to be ensured that the pipe parts are buttedagainst each other, that is to say bear closely or flush against eachother. Apart from the cylindrical fusion surface between socket innerwall and outer surface of the pipe parts covered by it, there isconsequently also a fusion jointing over the pipe wall thickness. As aresult, the heating element, whether provided in the form of a heatingcoil, a wire mesh or the like, is fully sheathed with plastic. Completeprotection against corrosion is consequently ensured. The medium laterflowing in the pipe consequently also cannot be contaminated bycomponent parts of the heating coil.

According to the invention, it is further provided that a bellows isarranged inside the plastics pipe parts in the region of the butt joint,is subjected to a pressure of about 1 to 3 bar and, after solidifying ofthe softened plastic, is relieved and removed from the jointed plasticspipe.

This bellows which is provided in the case of the process according tothe invention and, under increased pressure, bears against the innerwall of the pipe parts over at least the axial length of the socketprevents any escape of softened plastic into the pipe bore. Thisreliably avoids the formation of a disturbing inner bead. In addition,because it extends at least over the axial length of the socket, thebellows generally stabilizes the pipe parts to be jointed, and preventsany deformation of the thin-walled pipes. If the process parametersaccording to the invention are maintained, the conditions areconsequently particularly well suited also for fusion-jointed PVDF pipesby means of electric sockets.

According to the invention, the bellows which is used is provided atboth of its axial ends with in each case a ring of larger diameter thanthe bellows itself. These rings, which can be made of plastic, haveslightly smaller diameters than the pipe parts to be fusion-jointed.With appropriate material selection for the rings, the bellows can betransported very easily to the fusion joint. Tetrafluoroethylene issuitable as the material for the rings of plastic.

The invention is also directed at an electrically heatable or electricsocket for carrying out the process according to the invention, whichhas a heating element in the region of its inner wall.

The socket is characterized in that it has greater heating output in theregion of the butt joint to be covered than in the two outer regionsthereof. With the invention, increased heating output is introduced inthe region of the butt surface of both pipe parts.

This results in the softening or melting of the pipe ends, as a resultof which a butt fusion-jointing and continuous axial jointing of the twopipe parts occurs. This method of carrying out the inventionadditionally contributes achieving the goal of a corrosion-protectedarrangement of the heating element.

A preferred embodiment of the invention is distinguished in that theheating element is a heating wire coil which is wound more closely inthe region of the butt joint than at the two adjoining outer regions,thus having a smaller pitch in this region.

The heating element according to the invention may also be a heatinggrid or heating mesh, with wires or heating elements running radiallywith respect to the pipe parts being arranged closer together in theregion of the butt joint to be covered than in the two outer regions. Anincrease in the heating output in the region of the butt joint can alsotake place by the heating wire diameter being thinner in the region ofthe butt joint than in the two outer regions. Finally, according to theinvention, the goal of increased heating output can be achieved in that,with the same pitch of the heating coil over the socket, an additionalheating wire coil is provided in the region of the butt joint.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention emerge fromthe following description of preferred embodiments and with reference tothe attached drawings, wherein:

FIG. 1 shows a partial longitudinal section through two pipe parts to befusion-jointed with a conventional electric socket according to theprior art;

FIG. 2 shows the same view as FIG. 1 of an electric socket according tothe invention with the pipe parts to be fusion-jointed;

FIG. 3 shows a modified embodiment of the invention of FIG. 2;

FIG. 4 shows a representation of the preparation of the processaccording to the invention;

FIG. 5 shows a clamping device for carrying out the process according tothe invention, in plan view;

FIG. 6 illustrates fusion-jointing device for carrying out the processaccording to the invention, and

FIG. 7 shows the socket according to the invention as shown in FIG. 2,in an enlarged representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the prior art of the fusion jointing of two HDPE pipe parts1 and 2, which are positioned one adjoining the other while maintaininga gap 8. An electric socket 3 has in the region of its inner surface aheating wire coil 4. If an electric current is sent through this coil,resistance heating occurs, with the consequence that the HDPE parts ofthe socket 3 and of the pipe parts 1 and 2 adjoining the heating wirecoil 4 are incipiently melted, which results in a fusion-jointing of thepipe parts 1 and 2 respectively to the socket 3 upon cooling. In thecase of the known process, however, the gap 8 is not filled, and theheating wire coil 4 remains partially open with respect to the pipeinner surface and is consequently subject to corrosion.

The socket 3 according to the invention is particularly represented inFIGS. 2 and 3. In the case of the process according to the invention,the two pipe parts 1 and 2 are butted against each other, i.e. they bearagainst each other, as is indicated by the joint or butting surface 15.

The heating wire coil 4 is wound more closely in an inner region 5,which covers the joint 15, so that a more intense heat developmentoccurs in this inner region upon heating the heating wire coil than inits two adjoining outer or relatively wide regions 6. The heating can becontrolled or the arrangement of the coil windings can be made such thata complete softening of the pipe parts 1 and 2 occurs at the ends of thepipe parts 1 and 2 forming the joint 15, the two pipe parts 1 and 2being butt-fusion-jointed to each other after cooling down, in additionto the fusion jointing by means of the electric socket 3 along thecylindrical bearing surface of socket 3 and pipe parts 1 and 2. Beforethe beginning of the heating or welding operation, a bellows 10 may alsobe introduced into the pipe bore, essentially into the region covered bythe socket 3. It is subjected to pressure, preferably air pressure, at 1to 3 bar and consequently on the one hand supports the entire pipe partsheated by the socket and consequently preserves the circularcross-sectional shape of the pipes, since the admission of pressuremakes it bear closely against the inner wall of the pipe parts 1 and 2.In addition, it prevents any penetration of softened plastic into thepipe bore at the butt surface 15 of the two pipe parts 1 and 2, which atthis point are softened throughout, i.e. extending through the pipe wallthickness, by the heating output intensified there according to theinvention. In this way, the formation of an inner bead, disturbing forlater operation, is also reliably suppressed.

After cooling down of the softened plastic parts between the inner wallof the socket 3 and outer wall of the pipe parts 1 and 2 and also alongthe butting surface extending radially at the butting surfaces 15 of thepipe parts 1 and 2, the bellows 10 is removed from the pipe bore afterswitching off the heating of the socket. The pipe parts 1 and 2 arefusion-jointed to each other to form a single pipe, their circularcross-sectional shape being precisely preserved and an inner bead andgap 8 being avoided.

As FIG. 4 reveals, the bellows 10 may also be surrounded in the regionof its axial ends by rings of plastic, which preferably consist ofpolytetrafluoroethylene or a similar material which slides comparativelywell in the pipe parts 1 and 2 to be fusion-jointed. The outsidediameter of the rings of plastic 30 is slightly smaller than that of thepipe parts 1 and 2, into which the bellows 10 is to be pushed. Thebellows 10 is arranged with respect to the butt joint 15 such that therings of plastic 30 are arranged essentially symmetrically with respectto it. Then, the bellows 10 is subjected to pressure, it coming to bearalong its part located between the rings of plastic 30 closely againstthe butt joint 15 and, during the actual fusion-jointing operation,reliably counteracting the formation of a fusion bead.

FIG. 4 shows a diagrammatic representation of the preparation for thefusion-jointing operation. The socket 3 has already been pushed onto thepipe part 2. The pipe part 1 is subsequently likewise pushed into thesocket 3 from the side opposite the pipe parts 2, until it meets thepipe part 2 centrally in the socket. Both pipe ends are planed flatprior to the fusion jointing operation. Subsequently, the bellows 10 isintroduced into the pipe bore under the socket 3. In the case of theexemplary embodiment drawn, the bellows extends over an axial lengthwhich is approximately two times greater than the socket 3 andconsequently supports the fusion-jointing region of the socket and theadjoining regions of the pipe parts 1 and 2 reliably after pressureapplication. The pressure is supplied via a tube connection at the end11.

FIG. 5 additionally illustrates a clamping device 12, in which the pipeparts 1 and 2 are clamped axially in line during the fusion-jointingoperation. The bellows 10 is not shown in the case of this exemplaryembodiment. The pipe part 2 still has to be introduced into the socket 3by means of the clamping element 32 assigned to it.

The clamping device 12 basically comprises a rest 31, on which threeclamping elements 32 are mounted displaceably and fixably. One clampingelement 32 each serves for receiving the pipe part 1, the socket 3 andthe pipe part 2. These component parts of the pipe to be fusion-jointedare respectively able to be aligned and fixed in the clamping elements32 in line with one another, so that these elements are firmly arrangedwith respect to one another for the fusion-jointing operation.

In FIG. 3, a socket 3 corresponding to the socket 3 shown in FIG. 2 isrepresented. It differs from the socket 3 shown in FIG. 2 in that it isprovided on both sides with socket attachments 14 and is consequentlyaxially extended. Clamping device 12 can therefore be dispensed with incases of pipe parts of small diameter. The socket attachments 14 helpensure that the pipe parts 1 and 2 are reliably secured and arranged inthe actual socket 3 before the fusion-jointing, the jointing surfacesalso in this case butting one against the other and then forming thejoint 15.

In FIG. 6, a fusion-jointing generator 16 with bellows 10 and socket 3for carrying out the process according to the invention is represented.The bellows 10 is connected via the fusion-jointing generator 16 to acompressed-air source 38 and is then supplied with controlled pressure,via a line 39, according to programming by means of a bar code to beread in. The socket 3 is connected via lines 17, 18 to thefusion-jointing generator 16 and is supplied from there with electricpower.

The fusion-jointing generator is generally supplied with a voltage of110 to 220 volts. It supplies a fusion-jointing current with a voltageof about 5 to 10 volts and a current intensity of about 4 to 5 amperes.Depending on the data which are stored in the bar code, which issupplied along with the pipes or the sockets, the fusion-jointed timeand the fusion-jointed current are then automatically preset. Valuesconcerning the manufacturer, dimensions of socket and pipe, wallthickness and the consequently required fusion-jointing time may bestored in the bar code.

The invention shows a way of fusion-jointing comparatively thin-walledpipes made of PVDF and also of other plastics to each other. Thediameter of these pipes is generally about 20 to 160 mm. The wallthickness is about 1.9 to 10 mm. The wall thickness of the socketsdescribed here amounts to about 3 to 10 mm, that of the socketattachments to about 1.5 mm. The axial extent of the socket attachmentsin the preferred case is about 7 to 8 mm. In cases of pipe diameters of20 to 35 mm, the axial length of the socket according to the inventioncorresponds approximately to the pipe diameter; in cases of pipediameters of about 35 to 160 mm, about 0.6 to 0.4 times the diameter. Itfollows from this that the sockets according to the invention havecomparatively short axial lengths. In this way, it is possible tofusion-joint not only straight-extending pipes to each other, but alsopipe elbows and fittings each other and/or to straight pipe sections.

To illustrate the socket according to the invention, a preferredexemplary embodiment is once again represented, greatly enlarged, inFIG. 7. The two pipe parts 1 and 2, which are butted against each otheralong the butt joint 15, can be seen. The socket 3 is already providedsymmetrically over the butt joint. FIG. 7 illustrates the pipe parts 1,2 and the socket 3 in longitudinal section, only one half of thislongitudinal section being represented, as can be seen. In the presentcase, the socket comprises a sleeve 33, on which the heating coil 4 isprovided, to be precise in such a way that the winding spacings withrespect to one another are smaller in the region of the butt joint 15than in the two outer regions of the socket, so that in this region ofthe butt joint 15 to be covered there is in comparison with the outerregions an intensified heating output of the socket, which results alsoin the fusion-jointing of the butted-together end faces of the pipeparts 1 and 2 along the butt joint 15.

The sleeve 33 has in the region of its axial ends in each case anattachment 34, to which the two ends of the heating coil 4 are applied.Electric contact elements 35 are then fitted onto the attachments. Theseare operations in the production of the socket, which of course have totake place before the complete socket is arranged on the pipe parts 1and 2. The contact elements 35 are connected to the lines 17, 18.

In the actual production operation of the socket 3, after arranging theheating coil 4 and the terminals 35 in the way described, the unit thuscreated is encapsulated in plastic in a further operation and a furthermold, so that the final socket 3 is obtained in the form drawn. Aconnecting socket 36 is at the same time produced for each of theterminals 35.

It must further be emphasized that socket 3 which is particularlyprotected against contamination by the integrated heating coil 4 isobtained if, according to the invention, the heating wire from which theheating coil 4 is wound is coated with PFA before its integration intothe socket 3, that is to say before the winding-up onto the sleeve 33,and if the actual socket 3, including the sleeve 33, is formed fromPVDF. PFA has a somewhat higher melting point than PVDF. In thefusion-jointing operation, the sheathing of the heating wire istherefore softened less than the PVDF of the socket, so that any freematerial particles of the heating wire are held in the PFA sheathing.With such a socket, in an advantageous way, pipe parts 1, 2 made of PVDFare also fusion-jointed. It is sufficient if the PFA coating of theheating wire has a thickness of about 0.1 mm.

As evident from the foregoing description, a number of modifications notspecifically disclosed herein could be made to the device, but still bewithin the intended scope of the invention. To determine the scope ofthe invention, therefore, reference should be made to the appendedclaims.

I claim:
 1. A socket for fusion-jointing of plastic pipe parts, saidsocket comprising:a housing having two ends, an outer surface and innersurface; an electric heating element disposed in said housing adjacentthe inner surface thereof, said heating element being configured toprovide a higher heating output at a center region thereof between thetwo ends, adjacent a joint to be fused, than at outer regions thereoftoward the two ends and disposed away from the joint to be fused.
 2. Asocket as recited in claim 1, wherein said electric heating elementcomprises a heating coil, said heating coil having a winding spacingwhich is smaller in the center region adjacent the butt joint to befused than at the outer regions thereof.
 3. A socket as recited in claim1, wherein said electric heating element comprises a heating grid formedof a wire mesh, with wires of the mesh which are radially disposed withrespect to pipe parts to be joined are closer together in the centerregion adjacent the joint to be fused than the outer regions thereoftoward the two ends.
 4. A socket as recited in claim 1 wherein theelectric heating element comprises a coil having a constant pitch,wherein a diameter of heating wire forming the coil is thinner in thecenter region adjacent the joint to be fused than in outer regionsthereof.
 5. A socket according to claim 1, wherein said electric heatingelement comprises a first heating coil extending in an area between thetwo ends of the housing, with a second heating coil provided in thehousing in the center region adjacent the joint to be fused.
 6. A socketaccording to claim 1, wherein the housing has an axial length which isin a range of 0.2 to 1.00 times an inner diameter of one of the firstand second pipe parts.
 7. A socket according to claim 1, wherein thehousing has an axial length which is approximately equal to an innerdiameter of one of the first and second pipe parts.
 8. A socketaccording to claim 1, wherein the housing has an axial length which isin a range of 0.4 to 0.6 times an inner diameter of one of the first andsecond pipe parts.
 9. A socket according to claim 1, said socket furthercomprising holding means for holding the pipe parts to befusion-jointed, said holding means being integrally attached to the atleast two ends of the housing.
 10. A socket according to claim 9,wherein said holding means comprises socket attachments attached to saidtwo ends of the housing, said socket attachments having inner diameterswhich correspond to an inner diameter of the housing, said socketattachments having a wall thickness which is less than a wall thicknessof the housing.
 11. A socket as recited in claim 1, wherein saidelectric heating element is embedded within the socket housing.
 12. Asocket as recited in claim 1, wherein said electric heating element isdisposed in the housing between the two ends thereof.